Foreign genes are incorporated into viral DNA in insect cells using co-transfection of baculovirus DNA with a foreign gene-incorporated transfer vector into insect cells. As a result, foreign gene-encoding proteins are expressed on the budded virus envelopes from recombinant baculovirus (For example, Patent Document 1: JP 2003-52370 A). JP 2003-53270 A reported that G protein-coupled receptor-expressing budded viruses are prepared from their recombinant baculovirus and demonstrate a binding activity of hormone to the receptors on their envelopes.    [Patent Document 1] JP 2003-52370, A